Control system



- Nov. 9, 1943. BOWMAN 2,333,978

CONTROL SYSTEM Filed June 27, 1940 LEFT WIND I RIGHT WIND Inventor; Kenneth K, Bowman, y W

His Attorney.

the invention,

Patented Nov. 9, 1943 2,333,978 CONTROL SYSTEM Kenneth K. Bowman, Scotia, N. Y., assignor to General Electric Company,

New York a corporation Application June 27, 1940, Serial No. 342,769

11 Claims.

This invention relates to control systems, more particularly to systems for controlling the unwinding of a length of material from a reel, and the rewinding of the material on a second reel, and it has for an object the provision of a simple, reliable, inexpensive, and improved control system of this character.

Another object of the invention is the provision'of a control system of this character in which the conventional expensive, intricate and deli-- cate regulators are dispensed with and are supplanted by a single simple, rugged and inexpensive device which controls the relative field excitation of the dynamoelectric machines so as to maintain the tension in the material and the linear speed thereof substantially constant as the reel diameters vary.

A furtherobject of the invention is the provision of a control system of the character described in the foregoing in which tension is maintained in the material when the system is stopped or stalled and in which tension is not lost when the transfer from running tension to stalled tension is effected.

A still further object of the invention is the provision of means for reducing the stalled tension to a fraction of the running tension and for maintaining a predetermined proportionality between the stalled tension and the running ten.- sion.

A still further object of the invention is the provision of meansior eliminating tension variations in the material during acceleration and deceleration.

In carrying the invention into effect in one form thereof, a dynamoelectric machine is mechanically connected to each of the reels and these dynamoelectric machines are connected to a. source and energized therefrom in such a manner that one of said machines operates as a motor to wind the material, and the other operates as a braking generator driven by the material. In order to maintain the tension substantially constant, a device is provided which is responsive to the sum of the armature currents of the reel dynamoelectric machines for varying the relative field strength. of these machines as the reel diameters vary. In illustrating the invention in one form there-- of, it is shown as embodied in a reversing drive for a strip grinding and polishing machine. The invention of course is not limited to this particular application since it may be used in any material rewinding application.

For a better and more complete understanding or the invention, reference should now be had to the following specification and to the accompanying drawing in which Fig. 1 is a simple, diagrammatical illustration of an embodiment of and Fig. 2 is a diagrammatical sketch of a polishing machine which serves to illustrate one application or the invention.

Referring now to the drawing, a length of material Ill, such for example as a strip of'stainless steel, is unwound from a reel l l and passed through a set or idler rolls l2, l3, and I4 to a takeup reel l5 which is a duplicate of the payoff reel H. l

The grinding or polishing element, which drives and provides support and means for tension adjustment of the grinding or polishing abrasive belt l6 comprises the two idler rolls I1 and I8 and the driven roll IS. The roll I! may be driven by any suitable driving means such, for example, as an alternating induction current motor 20 which is supplied from a suitable source represented by the three supply lines 2 I. The grinding belt runs at a constant speed in one direction and is never reversed.

One of the idler rolls is so arranged and supported by means of a pneumatic cylinder (not shown) as to cause constant tension in the abrasive belt l6, and the amount of this tension is controllable by the operator.

As shown in Fig. 2, the abrasive belt l6 comes in contact with the material In at a point between the two idler rolls l3, and consequently, the material I0 is given a high polish as it passes from the payofl reel In to the takeup reel t5.

A dynamoelectric machine 22 is mechanically connected to the reel H and a similar dynamoelectric machine 23 is mechanically connected to the reel IS. The armatures of these machines are supplied from an adjustable voltage source such as the adjustable voltage generator 24. This generator is driven by any suitable driving means such as an alternating current induction motor (not shown) at a speed whichis preferably substantially constant.

The dynamoelectric machines 22 and 23 are provided with field windings 22a and 23a, respectively, which are connected in series relationship with each other to a source represented by the supply lines 25 and 26. The line 25 may be assumed to be the positive side of the source and the line 26 may be assumed to be the negative side of the source. The machines 22 and 23 are so connected to the adjustable voltage generator 24 and their excitations are so controlled that one of the machines tends to operate as a motor to wind up material on the takeup reel and the other machine operates as a braking generator driven by the payofi reel as the material is unwound therefrom. The operation of these two machines may be reversed to provide for reversing the direction of travel of the material for those cases in which it is desired to take two or more grinding passes on the strip of material without unloading or rethreading the reel.

- which is The generator 24 is provided with a field winding 24, which is arranged suitable source of excitation such, for example, as the source 25, 26 'by means Of the reversing contactors 21, 28. When the contactor 28 is closed, the field winding 24a of the generator is energized in such a direction that the dynamoelectric machine 23 operates as a motor to'wind up the strip on the reel l5, and the dynamoelectrio machine 22 operates as a generator driven by the unwinding reel H, and conversely, when the contactor 21 is closed, the dynamoelectric machine 22 operates as a motor and the dynamoelectrie machine 23 operates ator. A rheostat 29 is included in circuit with the field winding 24a for the purpose of adjusting the voltage of the generator 24 to any desired value. Since the speed of a motor may be varied by varying the voltage supplied to its armature, the linear speed of the strip of material may be adjusted to any desired value by means of the rheostat 29.

A booster generator 30 is connected in a loop circuit with the armatures oi the dynamoelectric machines 22 and 23. This booster generator is driven by any suitable driving means such as an induction motor (not shown) at a preferably substantially constant speed. The booster generator 33 .causes a current to fiow in the loop circuit which causes the dynamoelectric machines 22 and 23 to tend to operate as motors pulling in opposite directions on the strip thereby to maintain it "stalled" when the dynamoelectric machines 22 and 23 are disconnected from the supply generator 24. The current supplied to the loop circuit by the booster generator 33 is maintained at a low value which is less than that required by either of the dynamoelectric machines 22, 23 operating as a motor to develop a torque 'sufliciently great to overcome the friction of the system. Thus the dynamoelectric machine connected to the reel having the smaller diameter at the time is prevented from starting up as a motor and overhauling the dynamoelectric machine connected to the reel of larger diameter, and thus creepage is prevented.

One of the requirements of a winder drive is that both the strip tension and the strip speed should be adjustable through wide ranges. Another requirement of such a drive is that the strip speed and the strip tension should be maintained substantially constant at any values to which they are adjusted within these ranges In order to maintain both the strip speed and the tension of the strip substantially constant during a winding operation, it is necessary to decrease the speed 01 the dynamoelectric machine which is operating as a motor to drive the winding up reel, and this makes it necessary to strengthen the field oi the dynamoelectric machine operating as a motor and to weaken the field or the dynamoelectric machine which operates as a braking generator as the diameters of the reels vary. 7

For any given voltage applied to the armatures oi the dyna'moelectric mach nes 22 and 23, the tension .in the material will remain constant it the armature currents of the dynamoelectric machines 22 and 23 remain constant. It the voltage and current or the dynamoelectric machines 22 and 23 remain constant, the horsepower of each voi these machines will be constant and hence, it the horsepower of the machines is a function 01' the product or the tension and speed of the material, the speed 01' the material must also be constant as long as the'horsepower and to be connected to a as a braking genertension remain constant. ing the excitations of the field windings 22. and 23a of the dynamoelectric machines 22 and 23 to maintain the armature currents of these machines constant, the tension and linear speed of the material are also maintained constant.

For the purpose of controlling the excitation oi the field windings 22. and 23a, a special armature reaction excited dynamoelectric machine 3| is provided. Dynamoelectric machine 3| is driven at a speed which is substantially constant by any suitable driving means such, for example, as a synchronous motor (not shown). As shown in the drawing, the dynamoelectric machine 3| has two sets of brushes. One set of brushes 3|. is connected to an external load which in this case is the field winding 30. of the booster generator 30 and the field winding of whichever of the reel dynamoelectric machines 22, 23 is operating as a generator at the time. The other set of brushes 3h, is short circuited. The axis of the fiux which is produced by the current flowing in the short circuited armature conductors is .referred to as the short circuit axis, and the axis which is displaced 90 degrees from the short circult axis is referred to as the control axis. If it be assumed that the axis of brushes 3| coincides with the short circuit fiux axis, then the load brushes 3|. are on the control axis. The net flux along the control axis is produced by two opposing control field windings 3|e and 3ld, a series Consequently, by varycompensating field winding 3|; and the armature reaction of the load current. It is to be noted that the control field winding 3|s is connected across the voltgae drop resistors 32 and 33 which are connected in the armature loop circuit of the reel dynamoelectric machines 22 and 23. The fiux along the short circuit axis 01' dynamoelectric machine 3| is produced by the shunt field winding 3|: and the armature reaction of the short circuit current. This short circuit axis flux gencrates the voltage which appears across the load brushes 3|, and the control axis fiux produces the voltage which appears across the short circuited brushes 3| and causes short circuit current to flow.

The operation of the machine 3| will be readily understood by considering the operation without a second control field winding 3| and oi. the shunt and compensating fields 3|: and He, and then subsequently considering the efi'ect 01' these fields on the operation.

Voltage is applied to the control field winding 3|. and current begins to build up in this field winding. Immediately there is generated a voltage in the short circuit path which circulates a 1 large current in the short circuit.

Since the low as to be resistance of the short circuit is so almost negligible, the control field winding 3| is only required to produce a very small fiux in order to produce a large current in the short circuit and a correspondingly large armature fiux. Furthermore, since the fiux of the control field winding need only build up to such a low value, and since the reactance of the short circuited armature is very low, full load current in the short circuit will be obtained in an exceptionally short time. The armature reaction flux produced by the short circuit current generates a voltage across the load brushes 3|. and causes load current to flow. This load current will produce an armature fiux which opposes the flux of the control field winding 3|e. This would, of course, mean that the flux oi the control field winding 3| c would have to be large enough'to overpower this control axis armature flux. However, by using a series field winding 3h through which the load current passes, the armature reaction on the control axis can be completely neutralized. Thus, by the use of'this series compensating field, the control field is only required to produce enough field flux and voltage'to overcome the resistance drop in the short circuit, and the time constant or rate of response is very fast.

The function of the shunt field winding 3|: is to reduce the steady state short circuit current. For any given voltage across the load brushes 3h, there must be a corresponding flux on the short circuit axis. If any part of this flux is produced by a shunt field winding, the amount of flux which must be produced by the short circuit armature current will be correspondingly reduced. For example, if the shunt field is adjusted to give enough excitation to generate nine-tenths of the voltage across the load brushes 3h, the short circuit current for any given load voltage need be only one-tenth as great as it would have to be if there were no shunt field winding.

As pointed out in the foregoing, the two control field windings 3M. and 31d oppose each other.

The control field winding 3'0 is connected to the source 25, 26, and a variable resistor 34 and afixed resistor are included in the circuit. Field winding 3le causes the machine 3| to increase its output voltage and thereby to strengthen the excitation of the field winding of the reel dynamoelectric machine which is operating as a generator and to weaken the excitation of the field 'f'winding' of the reel dynamoelectric machine which is operating as a motor. Field winding 3hr which is excited in accordance with the armature current of the reel dynamoelectric machines tends to have the opposite efiect.

The variable resistor-'84 in the circuitof the control field winding 1310 servesto adjust the output voltage of the-machine 3| and thereby -to adjust the tension in the material In to a. value corresponding to the setting of the rheostat 34.

The fixed resistor 35 in the circuit of control field winding 3k is arranged to be short cireuited when either of the directional 'contactors 21 and 28 is closed, and thus field winding 31 has more effect on the operation of the machine 3! under running conditions. However, when both of the contactors 2'! and 28 are open, resistor 35 is inserted in the circuit of the field winding 31c and its efiect is to reduce the output voltage of the machine 3| and thereby to reduce the stalled tension in the material to a value which is proportional to the value of -the running tension.

With the foregoing understanding of the elements and their organization in the complete control system, th operation of the system itself will readily be understoodfrom the following detailed description:

Prior tothe starting of the grinding operation,- it may be assumed that the motor which drives the booster generator 30 is running andthat the booster generator 3|] is supplying a low value of current through the armature loop circuit of the reel dynamoelectric machines 22 and 23, and that these machines are accordingly maintaining stalled tension in the material I. It may also be assumed that the motor which drives the adjustable voltage supply generator 24 is run.- ning. However, since the field Winding 24a of the generator is disconnected from its source of excitation, the generator is not generating any voltage except the small voltage due to its residual magnetism.

Assuming that the reel H is full and the reel I5 is empty, the grinding operation will be carried out by unwinding the material Hi from the vthe closed position of this contactor, contacts 28a are closed to complete the connections, from the armatures of the reel dynamoelec'tric machines 22, 23 to the armature of the adjustable voltage supply generator 24.- It may be assumed that the terminal 24b of the generator 24 is the positive terminal and that the terminal 24.: is the negative terminal. The circuit is traced as follows: From the positive terminal 241, of the generator through the voltage drop resistor 32, the armature of the dynamoelectric machine 22, armature ot booster generator 30, contacts 28. bridged by the movable contact member to the negative terminal 24. The circuit for the machine 23 is traced from the positive terminal 24b through voltage drop resistor 33, through armature of reel dynamoelectric machine 23 and contacts 28a to the negative terminal 24a of the supply generator. The closing of contacts 281: and 28c completes the connections between the load brushes 3h; of dynamoelectric machine 3| and the field circuit of the dynamoelectric machines 22 and 23. This circuit is traced from the positive terminal 25 of the source of excitation through conductor 31, contacts 28b bridged by the movable contact member, conductor 38, through the armature of machine 3|, compensating field winding 31, conductor 39, contacts 280 bridged by the movable switch member and thence by conductor 40 to field winding 23a to the negative terminal 26 of the excitation source. It must be noted that the armature of the dynamoelectric machine 3| is connected in parallel with the field winding 22a. of reel .dynamoelectric machine 22 and in series with the field winding 23a. Contacts 28d being bridged by the movable contact member short circuit the resistor 35 which serves to limit the stalled tension in the material I0 and thus the effect of this resistor upon the tension during the subsequent grinding operation is removed, and the tension in the material during the grinding operation will be maintained at the value which corresponds to the setting'of the tension control rheostat 34.

The bridging of the stationary contacts 28 and 281 by their cooperating movable contact members completes the connections between the field winding 24a of the supply generator and the source of excitation 25, 26. The voltage of the generator 24 begins to rise in response to the energization of its field winding and as a result current is supplied to the armatures of the dynamoelectric machines 22 and 23, which begin to operate as motors and to accelerate to a speed across the voltage drop resistors 32 and33 in they unwinding reel machine 22 to losses under normal steady armature loop circuit of the reel dynamoelectric machines 22 and 23 is energized in accordance with the sum of the armature currents of the reel dynamoelectric machines and opposes the excitation of the control field 34s. This results in lowering the output voltage of the machine 35, which has the eifect of strengthening the excitation of the winding reel machine 23 and weakening the excitation of the unwinding reel machine 22 until a balanced condition is finally reached.

Under normal steady state operating conditions, the relative excitations of the field windings 22s. and 23a of the reel dynamoelectric machines as efiected by the machine 3! is such that the unwinding reel machine 22 tends to rotate at a slower speed than that of the winding reel machine 23. Consequently, the unwinding reel machine 22 is overhauled by the winding r'eel machine 23 and is caused to operate as a generator returning power to the winding reel machine 23. The direction of current flow in the loop circuit which results from the operation of the unwindin reel machine 22 as a generator is indicated by the arrows 22b and 23s. The current of the when operating as a braking generator is returned to the winding reel machine operating as a motor, and the supply generator 24 merely supplies the deficiency due state operating conditions.

As the winding of the material on the wind ing reel 55 progresses and the diameter of the coil increases, the tension in the material and the speed thereof and the load on the winding reel machine 23 tend to increase. As a result, the speed of the windin reel machine tends to decrease and its armature current tends to increase. Increased current flowing in the loop armature circuit of the dynarnoelectric machines and 23 increases the voltage drop across the voltage drop resistors 53% and and this results in strengthening the opposing control field ills of the dynamoelectric machine 3i, and this in turn results in decreasing the output voltage of the machine ill. Since the armature of the machine 3| is connected in parallel with the field winding 22a and in series with the field winding 232., the decreasing output voltage of the machine 3! results in weakening the excitation of the field winding 22a, and strengthening the excitation of the field winding 23a. The weakening of the field of the unwinding reel machine 22 decreases its counter voltage and thus decreases the c i:- rent flowing in the armature loop circuit. Likewise, the strengthening of the field of the winding reel machine 23 decreases its speed, which results in decreasing its armature current. The decrease of the armature current of the reel dynamoelectric machines 22 and 23 produces a corresponding decrease in the sum of the voltage drops across the voltage drop resistors 32 and 33, and this reduces the excitation of the opposing field 3ls of the dynamoelectric machine 3i until the original balance is reestablished. When the original balance is reestablished, the tension in the material of course is restored to its original value.

Although the operation of the dynamoelectric machine 3| in maintaining the tension in the material constant at a predetermined value is described as taking place in a series of steps, it will be understood of course that this entire operation is extremely rapid and that it is changing quently, as the voltage of the generator asaae'rs continuously as the diameters of the reels change throughout the winding operation.

From the foregoing description it will be noted that the dynamoelectric machine 3! functions to maintain the sum of the voltage drops across the resistors 32 and 33 constant. The maintaining of the sum of the voltage drops across the resistors 32 and 33 constant results in maintaining the armature currents of the reel dynamoelectric machines 22 and 23 constant during normal steady state winding operation, and this results in maintaining the tension in the material constant provided the voltage of the generator 24 does not change. However, during acceleration from rest to any preset speed or from one preset speed to another, it is necessary for the armature current in the reel machines to change, i. e., the armature current in the reel machine that is operating as a motor must be increased and the armature current of the reel machine that is operating as a braking generator must be decreased. If the dynamoelectric machine 3| were arranged to maintain constant armature current in each of the reel dynamoelectric machines 22 and 23, acceleration could not take place without varying the tension of the material which would be undesirable. This undesirable operating condition is eliminated by making the voltage drop resistors 32 and 33 of equal ohmic value and connecting the control field winding Bid or the dynarncelem trio machine 35 across these resistors. Thus, during acceleration when the voltage of the gen erator 25 is increased, the armature current of the machine 23 which is operating as a motor in creases. The voltage of the machine 22 is greater than the voltage of the generator 25 because the machine 22 is operating as a generator. Conse is in creased, the difference between the volta e of the machine 22 and the generator is decreased and therefore the armature current of the machine 22 is correspondingly decreased. This is a condi-- tion which is necessary to effect acceleration without varying the tensln or" the strip, and the operation of the dynamoelectric machine 3i does not interfere with the establishment of this necessary condition. The reel dynamoelectric machines 22 and 23 are as nearly identical in design and in operating characteristics as it is possible to construct them and since the resistors 32 and 33 are of equal value, the amount of increase of armature current of the machine 23 will be exactly equal to the amount of armature current decrease of the machine 22. Consequently, the increase in voltage drop across the resistor 33 will be exactly equal to the decrease in voltage drop across the resistor 32 so that the sum of the voltage drops across these two resistors will remain the same. As a result there will be no change in the excitation of the field winding is of dynamoelectric machine 33, and therefore, machine 3! will not change the relative excitations of the field windings 22a and 239, during acceleration.

It is possible to design the dynamoelectric machine 3| so that the excitation of its control field winding 3h may be adjusted to hold any value of tension in the material It! Within a range of 10 to 1. Frequently, however, a much greater range, such as 25 to l, of tension variation is required for a particular installation. This increased range may be provided by means of switches H and 42 arranged to short circuit equal portions of the voltage drop resistors 32 and 33. For ex-- ample, when the switches 4i and 42 are closed to short circuit the upper portions of the resistors condition in which constant tension is maintained until the armature current of the reel dynamo electric machines 22 and 23-increases to a value at which the sum of the voltage drops across the.

unshort circuited portions of resistors 32 and 33 attains a value equal to the sum of the voltage drops across resistors 32 and 33 when no portion thereof is short circuited. In other words, the armature currents of the reel dynamoelectric machines 22 and 23 must; be increased in order to produce a balanced operating condition, and this results in correspondingly increased tension.

The winding operation is terminated by moving the switch 36 from the upper position to its intermediate position in which it is illustrated. This results in deenergizing the contactor 28 which opens its contacts 28p. to disconnect the reel dynamoelectric machines from the adjustable voltage supply generator 24. When the reel dynamoelectric machines 22 and 23 are disconnected from the generator fljthe booster generator 30 maintains stalled tension in the material III in the manner described in the foregoing.

The operation can be reversed and the strip of said reels, each of said dynamoelectric machines having a field winding, and means comprising an exciter-provided with a control field winding excited in accordance with the sum of the armature currents of said machines for varying the relative excitations of said first and second dynamoelectric machines so as to maintain the tension of said strip substantially constant as 1 the diameters of the coils on said reels vary.

length of material from a coil and the rewindin 3. A system for controlling the unwinding of a of said material to form a second coil comprising 3 in combination, a pair of reelsa'd'apted to be unwound from the now full reel l5 and rewound on the now empty reel I i by moving the switch 36 to its lower position in which the left wind directional contactor 21 is picked up and its contacts closed. This results in connecting th field winding 24,; of the supply generator to the source of excitation for reverse polarity which,,in turn, results in reversing the direction of rotation of the reel dynamoelectric machines 22 and 23. The en-- sui'ng operation in the reverse direction is otherwise identical with 'that described in the foregoing and consequently a repetition is unnecessary.

Although in accordance with the patent statutes this invention is described as embodied in concrete form and the principle thereof has been described together with the best mode in which it is now contemplated applying that principle, it will be understood that the apparatus shown and described is merely illustrative and that the invention is not limited thereto, since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A system for controlling the unwinding of a the provisions of of said material to form a second coil comprising joined by a length of material, a first dynamoelectric machine connected to one of said reels, a second dynamoelectric machine connected to the second of said reels, each of said dynamoelectric machines having a field winding and said field windings being connected in series relationship to a source, and means comprising an auxiliary dynamoelectric machine connected in par allel with one of said field windings for strength ening the excitation of one of said field windings and weakening the excitation of the other of said field windings as the diameters of the coils on said reels vary.

4. A system for controlling the unwinding of a length of material from a coil and the rewinding of said material to form a second coil, comprising in combination, a pair of reels adapted to be joined by a length of material, a dynamoelectric machine connected to one of said reels, 9. second dynamoelectric machine. connected to the other of said reels, each of said machines having a field winding and said field windings being connected strengthening the field of the machine operating in combination, a pair of reels adapted to be joined by a-length of material, a dynamoelectric machine mechanically connected to a first of said reels, 9. second dynamoelectric machine connected to the second of saidreels,each of said machines having a field winding, connections from a source of supply to the armature of said machines, and

means responsive to the sum of th armature currents of said machines for controlling the excitation of said field windings so as to maintain the tension of said material substantially constant as the diameters of the coils on said reels vary.

2. A system for controlling the unwinding of a length of material from a coil and rewinding said' material to form a second coil comprising in combination, a pairof reels adapted to be joined by a length of material, a first dynamoelectric machine connected to a first of said'reels, a second as a motor and for weakening the field of the machine operating as a generator as the diameters of the coils on said reels vary.

5. A system for controlling the unwinding of a length of material from a coil and the rewind-mg of said material to form a second coil comprising in combination, a pair of reels adapted to be joined by a length of material, a dynamoelectric machine mechanically connected to one of said reels, a second dynamoelectric machine mechanically connected to the other of said reels, each of said dynamoelectric machines having a field winding, a source of supply and connections from said machines to said source so that one of said machines operates as a motor to wind said material and the other operates as a braking generator driven by the material and means for strengthening the field of the machine operating as a motor and weakening the field of the machine operating as a generator as the diameters of the coils on said reels vary comprising an auxiliary dynamoelectric machine provided with a control field winding excited by the armature currents of both said first and second dynamoelectric machines.

6. A system for controlling the unwinding of a dynamoelectric machine connected to the second reels, a second dynamoelectric machine mechanically connected to the other of said reels, each of said machines having a field winding and said field windings being connected in series relation ship to a source of excitation, connections from saiddynamoelectric machines to a source of supply so that one of said machines is caused to opcrate as a motor to wind said material and the other is caused to operate as a braking generator driven by said material, and means for strengthening the field of the machine operating as a motor and for weakening the field of the machine operating as a generator to maintain the tension in said material substantially constant as the diameters of the coils on said reels vary comprising a dynamoelectric machine provided with a control field Winding connected to be excited by the armature currents of said first and second dynamoelectric machines in accordance with the algebraic sum of said currents.

7. A system for controlling the unwinding of a length of material from a coil and rewinding the material to form a second coil comprising cornbination, a pair of reels adapted to be joined a length of material, a dynamoelectric machine mechanically connected to one of said reels, a second dynamcelectric machine connected to the other of said reels, each of said dynamoelectric machines having a field winding, a source or" supply, electrical connections from said source to said machines so-that one of said machines is caused to operate as a motor to wind said material and the other machine is caused to operate as a braking generator driven by said mate ial, voltage drop device connected in the armatur circuit of each of said machines, said voltage drop devices being connected between one terminal of each of said machines and the same side of said source, and means for controlling the excitation of said field windings so as to maintain the tension in said material substantially constant comprising a dynamoelectric machine having a control field winding connected across said voltage drop devices and having its armature connected to the field windings of said first and second dynamoelectric machines.

8. A system for controlling the unwinding of a length of material from a coil and the rewinding of said material to form a second coil comprising in combination, a pair of reels adapted to be joined by a length of material, a dynamoelectric machine connected to one of said reels, a second dynamoelectric machine connected to the other of said reels, each of said dynamoelectric machines having a field Winding and said field windings being connected in series relationship to a source of excitation, an adjustable voltage source connected to said machines so that one of said Inachines operates as a motor to wind up the material and the other operates as a braking generator driven by the material, a voltage drop device connected in the armature circuit of each of said machines, said voltage drop devices being connected between one terminal of said machines and the same side of said source, and means responsive to the voltage drop across said devices for controlling the excitations of said field windings to maintain the tension of said material sub-- stantially constant.

9. A system for controlling the unwinding of a length of material from one coil and the rewinding of said material to form a second coil comprising in combination, a pair of reels adapted to be joined by a length of material, a first dynamoelectric machine mechanically connected to one of said reels and a second dynamoelectric machine mechanically connected to the other of said reels, each of said dynamoelectric machines having a field winding and said field windings being connected in series to a source of excitation, a source connected to said machines for causing one of said machines to operate as a motor to wind said material and the other to operate as a braking generator driven by said material, and means for controlling the excitation of said field windings to maintain the tension of said material substantially constant comprising an exciter having its armature connected in parallel with the field winding of one of said dynamoelectric machines and having a control field winding connected to be responsive to the sum of the armature currents of said machines.

10. A system for controlling the unwinding of a length of material from a coil and the rewinding of said material to form a second coil comprising in combination, a pair of reels adapted to be joined by length of material, a dynamoelectric machine mechanically connected to one of said reels, a second dynamoelectric machine connected to the other of said reels, each of said dynamoelectric machines having a field winding, a source connected to said machines to cause one of said machines to operate as a motor to wind said material and the other of said machines to operate as a braking generator driven by said material, and means for controlling the excitation of said field windings to maintain the tension of said material constant as the diameters of the coils on said reels vary comprising an armature reaction excited dynamoelectric machine having a pair of short circuited brushes and a pair of load brushes on an axis at an angle with the axis of said short circuited brushes and connected to the field windings of said first and second dynamoelectric machines and having a control field winding energized in accordance with the sum of the armature currents of said reel dynamoelectric machines.

ii. A system for controlling length of material from a coil and rewinding said material to form a second coil comprising in combination, a pair of reels adapted to be joined by a length of material, a dynamoelectric machine the unwinding of a connected to one of said reels, a second dynamoelectric machine connected to the other of said reels, each of said dynamoelectric machines having a field winding, an adjustable voltage source connected to said dynamoelectric machines to cause one of said machines to operate as a motor to wind said material and the other of said machines to operate as a braking generator driven by the material, a resistor connected in the armature circuit of each of said dynamoelectric machines, said resistors being connected between corresponding armature terminals of said dynamoelectric machines and one terminal of said source, means for varying the relative field strengths of said field windings to maintain the tension of said material substantially constant as the diameters of the coils on said reels vary comprising a control dynamoelectric machine having its armature terminals connected to the field windings of said reel dynamoelectric machines and having a control field winding connected to be responsive to the voltage drops across said resistors, and a second field winding arranged on winding thereby to adjust to be held in said material.

, mNNETlf-I K. BOWMAN. 

